Synthesis And Electrochemistry Performance Of Fluorine-free Ti₃C₂T_x（T=-OH,-O）

As a new type of two-dimensional material with high conductivity,MXene is widely used in electrochemical fields,such as lithium batteries,supercapacitors,electrocatalysis,etc.At present,the main synthesis method of MXene is acid etching（HF or HCl+Li F）and MXene is obtained via selectively etching A from the the raw material-MAX phase.Although effective,the acid etching method still has some disadvantages.First,acid cannot etch the MAX phase where A is an acidic element,which leads to the limitations of MXene type.Second,the fluorine groups introduced during the preparation process form M-F bonds on the surface of the material,making it difficult for the pseudocapacitor active site M to provide capacity through valence state changes,while-F also reduce the life of the electrode.Third,the use of acid reagents can cause environmental pollution.How to overcome the shortcomings of the existing preparation methods,constructing a new preparation strategy of MXene has become the focus of the current research.Therefore,taking a typical MXene material Ti₃C₂T_x as a research object,this thesis proposes a method for preparing fluorine-free Ti₃C₂T_x by hot-alkali etching and the supercapacitors and nitrogen reduction performance of fluorine-free Ti₃C₂T_x（T=-OH,-O）were studied.The main results are as follows:（1）Invented hot alkali etching method to prepare fluorine-free MXene.By calculating the thermodynamics of the chemical equation,the possibility of preparing Ti₃C₂T_x by alkali etching was determined.Inspiring from Bayer process we confirmed that the insoluble aluminum oxides and hydroxides produced in the reaction are the key to the success of alkaline etching Combined with the Bayer process conditions,a preparation route（270°C,27.5 M Na OH solution,argon atmosphere）that can successfully obtain high purity（92 wt%）fluorine-free Ti₃C₂T_x（T=-OH,-O）was set up.Comparing the obtained multi-layer fluorine-free Ti₃C₂T_x with fluorine-containing Ti₃C₂T_x,it is found that the fluorine-free Ti₃C₂T_x possess a relatively compact layered structure with more cracks on the surface.In terms of structure and element composition,characterization results proved that fluorine-free Ti₃C₂T_x（T=-OH,-O）was intercalated with sodium ions during preparation,and the resulting fluorine-free Ti₃C₂T_x has a larger interlayer distance.（2）The mechanism and characteristics of multilayer fluorine-free Ti₃C₂T_x（T=-O,-OH）in electrochemical energy storage were studied.The obtained powder was prepared as a supercapacitor electrode and supercapacitor performance was tested using a three-electrode test method.At a scan speed of 2 m V/s,the mass specific capacity of multilayer fluorine-free Ti₃C₂T_x（T=-OH,-O）in a 1 M H₂SO₄electrolyte is as high as 314 F/g（thickness 52μm）,improved by～214%than the reported capacitance performance of the fluorine-containing Ti₃C₂T_x electrode.The volume specific capacity of the fluorine-free Ti₃C₂T_x electrode is 511 F/cm³,which is equivalent to that of the fluorine-containing Ti₃C₂T_x electrode with the best performance reported in the same period.Besides,the fluorine-free Ti₃C₂T_x electrode has a capacity retention rate of 89.1%after 10,000 cycles under a discharge condition of 5 A/g,which is similar with fluorine-containing Ti₃C₂T_xelectrode.The results above confirmed that the fluorine-free preparation strategy can greatly improve the electrochemical performance of MXene.（3）The intercalation and delamination method of multilayer fluorine-free Ti₃C₂T_x and the corresponding performance of nitrogen reduction and ammonia production were studied.Due to the high temperature hydrothermal conditions in the preparation process,many defects were introduced on the surface of fluorine-free Ti₃C₂T_x,and the obtained fluorine-free Ti₃C₂T_x showed a few layers（2-5 layers）with a size of～50-300 nm.The size of nanosheets is 2-5 times smaller than the product of fluorine-containing Ti₃C₂T_x under the same conditions.It is showed that the fluorine-free Ti₃C₂T_x nanosheets stripped by TMAOH have the highest yield and the smallest size（～50-100 nm）during the 4selected reagents and showed an ammonia gas production of 36.9μg h^-1mg^-1cat and a Faraday efficiency of 9.1%at-0.3 V vs.RHE.Compared with fluorine-containing Ti₃C₂T_x,the ammonia gas production has doubled.Besides,due to the characteristics of fluorine-free and small size,fluorine-free Ti₃C₂T_x electrode showed smaller charge transfer resistance and larger electrochemical specific surface area.In summary,this study breaks the barrier of traditional alkali etching method containing fluorine,and provides a powerful theory basics and application potential for the subsequent creation and application of more fluorine-free MXene.